Methods of lossy image compression are known, both in the context of video compression and for individual images. The dominant examples are the methods defined in the various MPEG and JPEG compression/decompression standards, which include coarse quantization of image data. These lossy compression methods introduce artefacts, i.e. differences between original images and reconstructed images that are obtained after compression and decompression.
A method of reducing these differences has been described in a publication titled “Post filter SEI message for 4:4:4 coding” by S. Wittmann and T. Wedi, at the 18th meeting of the Joint Video Team (JVT) of ISP/IEC MPEG & ITU-T VCEG (document number JVT S030).
In this publication it is proposed to add messages to an MPEG video stream that define the filter coefficients for a FIR (Finite Impulse Response) post-filter that is to be applied to decoded images on the decoder side in order to reduce compression artefacts. Wiener filtering is used, that is, a statistical measure for the difference between the original image and the post-filtered, decompressed image is defined and the filter coefficients selected so that they minimize the value of this measure. As may be noted, this means that the value of the filter coefficients depends on the content of the image. The filter coefficients are computed in the encoder and included in the video stream. The decoder extracts the coefficients from the stream and applies post-filtering using the extracted coefficients.
Unfortunately, this type of post-filtering only provides a limited reduction of compression artefacts.